1. Field of the Invention
The present invention relates to a molding die and marking method for fabricating a resin mold package with a marked surface for semiconductor device.
2. Description of the Related Art
Conventionally, a light-coupled element as shown in FIG. 13 has been widely used as a kind of semiconductor element sealed in a resin mold package. Before assembling the light-coupled element as shown in FIG. 13, a light-emitting element 3 and a photo-detector 4 are coupled by a die bond to lead frames 1, 2, respectively. Further, the lead frames 1, 2 are wire-bonded to the light-emitting element 3 and the photo-detector 4, respectively, using bonding wires 5 of gold or the like, and a precoat resin 6 of silicon resin or the like is coated around the light-emitting element 3. After that, the lead frames 1, 2 are spot welded or set in a loading frame so that the light-emitting element 3 and the photo-detector 4 are placed in opposed relation to each other and held in such relative positions as to be coupled optically. Under this condition, the assembly is sealed with a primary mold resin 7 of light-transmitting epoxy resin or the like. Further, the resulting assembly is sealed with a secondary mold resin 8 of a light-shielding epoxy resin or the like.
The transfer molding is used for sealing with the primary mold resin 7 and the secondary mold resin 8. After molding, the portions of the lead frames 1, 2 exposed to the outside of the mold resin are subjected to a post-treatment such as exterior plating or forming. Further, the electrical characteristics of the assembly are checked as a light-coupled element, and marking a surface of the resin mold package with a mark is carried out.
FIG. 14 shows the steps of fabricating the light-coupled element shown in FIG. 13. Steps a1 to a3 represent the pretreatment of the light-emitting element 3, and steps b1 and b2 the pretreatment of the photo-detector 4. Steps c1 to c7 represent the process for fabricating the light-coupled element.
In steps a1 and b1, the light-emitting element 3 and the photo-detector 4 are die-bonded at predetermined positions of the lead frames 1, 2, respectively. The "light-emitting element" and the "photo-detector", which constitute an infrared "light-emitting diode" and a "photo transistor", respectively, for example, which will hereinafter sometimes be abbreviated as "LED" and "PT", respectively. In steps a2 and b2, predetermined portions of the lead frames 1, 2 are wire-bonded with bonding pads formed on the light-emitting element 3 and the photo-detector 4, respectively. The light-emitting element 3 is further precoated with the precoat resin 6 in step a3.
In step c1, the lead frame 1 on which the light-emitting element 3 is mounted and the lead frame 2 on which the photo-detector 4 is mounted are set in a molding die in such a manner that the light-emitting element 3 and the photo-detector 4 are optically coupled to each other, and the primary molding is performed using a primary molding die. In step c2, the secondary molding is performed by setting the mold of the primary mold resin 7 and sealing the mold with the secondary mold resin 8. Upon completion of the secondary molding, the portions of the lead frames 1, 2 exposed to the outside of the resin mold package are exterior-plated or formed by bending, followed by step c3 in which the electrical characteristics are checked. In the case where the assembly is determined as conforming as a result of the characteristics check, marking the surface of the resin mold package with the mark 9 is carried out in step c4. Then, in step c5, the appearance inspection is conducted on the parts including the mark. A conforming product is packaged in step c6, and shipped to the market or customers in step c7.
Light-coupled elements include a photo-coupler and a photo-thyristor. The feature of the light-coupled elements is that they are molded with the primary mold resin 7 such as a light-transmitting epoxy resin or the like in the primary molding process of step c1 of FIG. 14 and then molded with the secondary mold resin 8 such as a light-shielding epoxy resin. The resin mold package for sealing individual semiconductor elements, such as transistors or diodes, or the resin mold package for sealing a semiconductor integrated circuit requires no light-transmitting primary mold resin, and therefore the transfer molding is carried out only once. The process of step c3 and subsequent steps in FIG. 14, however, is basically the same.
A conventional method in which a molding die for semiconductor devices has a marking section so that the marking and molding are carried out at the same time is disclosed in Japanese Unexamined Patent Publication JP-A-6-120281(1994). In the marking section according to this prior art, forward ends of pin-shaped members are protruded from or recessed into the surface of the molding die thereby to form protrusions and recesses corresponding to the markings over the entire surface.
In the conventional process for fabricating semiconductor elements such as light-coupled elements, marking the surface of the package with a mark indicating a product name, a date of fabrication, a trade mark, etc. is carried out by stamping or radiation of laser beam in a separate marking step after molding. This additionally-required step deteriorates the fabrication efficiency and increases the management cost. Also, the mark is often erased or fouled by the dust on the package surface, displaced due to the positioning inaccuracy of the mold, or other inconveniences occur. The resulting lower yield and the increased need of labor for repair work cause a higher cost.
Once protrusions and recesses corresponding to contents of marking are formed directly on the molding die, the molding and marking can be carried out at the same time, thereby avoiding the inconveniences and the increased cost described above. For a semiconductor device including a light-coupled element, however, the same molding die may be used for a plurality of product lines or for the same product line with different markings indicating the date of fabrication and the production lots, in which case changes are needed in the contents of marking. In the case where protrusions and recesses corresponding to the contents of marking are formed directly on the molding die, the die must be replaced each time the contents of marking are changed, thereby leading to a deterioration of the operating efficiency and an increased cost.
A conventional method for forming protrusions and recesses directly on the molding die is proposed in Japanese Unexamined Patent Publication JP-A-6-120281(1994), in which protrusions and recesses are formed by a multiplicity of pin-shaped members protruded and recessed differently. Nevertheless, the die structure is so complicated that the die cost increases, resulting in an increased cost of the final product.